Specific involvement of caspases in the differentiation of monocytes into macrophages

Caspases are cysteine proteases involved in apoptosis and cytokine maturation. In erythroblasts, keratinocytes, and lens epithelial cells undergoing differentiation, enucleation has been regarded as a caspase-mediated incomplete apoptotic process. Here, we show that several caspases are activated in human peripheral blood monocytes whose differentiation into macrophages is induced by macrophage colony-stimulating factor (M-CSF). This activation is not associated with cell death and cannot be detected in monocytes undergoing dendritic cell differentiation in the presence of interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The mechanisms and consequences of caspase activation were further studied in U937 human monocytic cells undergoing phorbol ester-induced differentiation into macrophages. Differentiation-associated caspase activation involves the release of cytochrome c from the mitochondria and leads to the cleavage of the protein acinus while the poly(ADP-ribose)polymerase remains uncleaved. Inhibition of caspases by either exposure to the broad-spectrum inhibitor benzyloxycarbonyl-Val-Ala-_DL-Asp-fluoro-methylketone (z-VAD-fmk) or expression of the p35 baculovirus inhibitory protein or overexpression of Bcl-2 inhibits the differentiation process. In addition, z-VAD-fmk amplifies the differentiation-associated production of radical oxygen species in both phorbol ester-differentiated U937 cells and M-CSF-treated monocytes, shifting the differentiation process to nonapoptotic cell death. Altogether, these results indicate that caspase activation specifically contributes to the differentiation of monocytes into macrophages, in the absence of cell death.